Group-III element nitride compound semiconductors, such as gallium nitride (GaN) and the like, have attracted attention as materials for semiconductor devices that emit blue or ultraviolet light. Blue laser diodes (LDs) are applied to high-density optical discs and displays, and blue light emitting diodes (LEDs) are applied to displays, lights and the like. Ultraviolet LDs are expected to be applied to biotechnology and the like, and ultraviolet LEDs are expected to provide ultraviolet light for fluorescent lamps.
A substrate made of a group-III element nitride compound semiconductor, such as gallium nitride (GaN) or the like, for an LD or an LED is typically produced by heteroepitaxially growing a group-III element nitride crystal on a sapphire substrate using vapor phase epitaxy. Examples of vapor phase epitaxy include Metal Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase Epitaxy (HYPE), Molecular Beam Epitaxy (MBE), and the like. However, the dislocation density of a gallium nitride crystal obtained by these vapor phase epitaxy methods is 108 cm−2 to 109 cm2, which is poor crystal quality. To avoid this problem, ELOG (Epitaxial Lateral Overgrowth) has been developed, for example. This method can reduce the dislocation density to about 105 cm−2 to 106 cm2. However, this method disadvantageously includes a complicated step.
On the other hand, crystal growth may be carried out in liquid phase instead of vapor phase epitaxy. Since the nitrogen equilibrium vapor pressure at the melting point of a group-III element nitride single crystal, such as gallium nitride (GaN), aluminum nitride (AlN) or the like, is 10000 atm or more, liquid phase growth of a group-III element, such as a gallium nitride crystal, an aluminum nitride crystal or the like, needs to be conducted under severe conditions, such as at 1200° C. at 8000 atm (8000×1.013×105 Pa).
To solve this problem, a method of using an alkali metal, such as sodium (Na) or the like, as a flux has been recently developed. This method allows a group-III element nitride crystal, such as a gallium nitride crystal, an aluminum nitride crystal or the like, to be obtained under relatively mild conditions. As an example, in a nitrogen gas atmosphere containing ammonia, sodium (alkali metal) and gallium (group-III element) are melted by application of pressure and heat, and the melt (sodium flux) is used to conduct crystal growth for 96 hours to obtain a gallium nitride crystal having a maximum crystal size of about 1.2 mm (see, for example, Patent Citation 1). Also, a method in which a reaction vessel and a crystal growth vessel are separated and a large crystal is grown while suppressing spontaneous nucleation has been proposed (see, for example, Patent Citation 2). Also, a method of growing a high-quality bulk crystal, where an alkaline earth metal or the like is added to sodium, has been proposed (see, for example, Patent Citation 3).
A gallium nitride crystal obtained by the method of using a sodium flux has a low dislocation density (i.e., high quality), but a low growth rate (i.e., poor productivity) as compared to when vapor phase epitaxy is employed. Therefore, an improvement in growth rate is required for the method of producing a gallium nitride crystal in liquid phase, where an alkali metal, such as sodium or the like, is used as a flux.
Patent Citation 1: JP 2002-293696 A
Patent Citation 2: JP 2003-300798 A
Patent Citation 3: WO04/013385